Spin-on filters have been employed in a variety of applications including hydraulic systems and engine lubrications systems. Such filters generally include a filter element within a housing with a cover or nut plate secured at one end of the housing by which the filter can be screwed onto or off of a filter head. A central opening and several surrounding openings are provided in the cover to direct flow through the filter and filter element therein. In an inside/out flow arrangement, the fluid enters the central opening and exits through the surrounding openings after passing through the filter element within the housing. In an outside/in flow pattern, the fluid enters the surrounding openings and exits through the central opening after passing through the filter element. A circular gasket is provided on a top surface of the cover to serve as a seal between the filter and the filter head. A spring is often provided in the lower end of the housing to maintain the filter element in sealing engagement with the cover.
Although satisfactory in low and medium applications, generally spin-on filters of the prior art have not been satisfactory for use in high-pressure applications such as in hydraulic transmission pumps, where surges of 1000 psi or more can occur. Most spin-on filters currently available include covers constructed of a stamped steel disk, and a relatively thinner secondary disk spot welded thereto. The base disk includes an extruded, relatively shallow, internally threaded neck portion by which the filter can be connected to a filter head. Flow openings are punched into the base disk around the neck portion. The lip at the open end of the housing is connected, by means of a rolled lock seam, to the periphery of the secondary disk which is also formed to serve as a seat for the external gasket. In this design, fatigue failure is most likely to occur at the rolled lock seam or at the spot welds. A burst failure is most likely to occur either upon bending of the cover (which allows leakage past the external gasket) or upon unfolding of the rolled seam. Thus, prior art spin-on filters have been susceptible to failure at the cover and/or at the connection between the cover and housing. Welding of the housing and cover is often unacceptable due to the incompatibility of housing and cover materials such that a satisfactory weld cannot be formed.
U.S. Pat. No. 4,369,113 to Stifelman discloses a high strength spin-on filter in which a cover is inserted into the open end of a housing, and the open end of the housing is folded inwardly and back on itself over the peripheral rim of the cover to form a folded connection between the cover and housing. The cover includes a plurality of projecting rib portions which deform the housing at selected locations, such that relative rotation between the cover and housing is prevented. In addition, a somewhat curved portion or shoulder (often referred to as a bead) is provided in the housing sidewall upon which the cover sits to properly locate the cover prior to folding the housing open end over the cover. However, the deformation of the housing sidewall for anti-rotation purposes and for locating the cover result in stress concentrations which can result in failure, particularly after extended use. More significantly, special tooling is required for controllably folding the housing open end over the cover periphery and back upon itself. Such a folding operation is costly and time consuming since it requires both a 180.degree. and a 90.degree. folding of the circumference of a tubular housing rim.
U.S. Pat. No. 4,743,374 to Stifelman discloses a high-strength filter in which the open end of the housing is folded over the periphery of the cover member and into a groove formed in the top of the cover. The groove includes a plurality of projecting members which extend into the groove and engage with the housing to prevent relative rotation between the housing and the cover. As with the '113 device, special tools are required to fold the housing both over the periphery of the cover, and back on itself. Such a folding operation cannot be accomplished by a spin-seaming technique, which is often preferred due to the simplicity and rapid production time associated with spin-seaming. In addition, the '374 device also utilizes a shoulder or bead portion located below the cover for properly locating the cover within the housing prior to the folding operation. The '374 device also requires a complicated cover member in requiring both a top groove and projecting portions formed within the grooves Thus the '374 device requires die casting and machining of the cover which is not always possible and/or practical.
U.S. Pat. No. 4,834,885 to Misgen et al. discloses a seal arrangement for fluid filters in which the housing is folded over the cover and the gasket seal (forming the seal between the filter and filter head) is placed over the folded-over portion An additional folding step then folds an additional portion of the housing upon the side of the sealing gasket. However, such an arrangement requires a complicated folding operation in interconnecting the housing with the cover and the sealing gasket. In addition, in the Misgen et al. arrangement, the housing sidewall is deformed for anti-rotation purposes, which can lead to localized weakened portions and/or stress concentrations in the housing sidewall. Misgen et al. also utilizes a bead or shoulder portion located below the cover for seating the cover prior to closing of the open housing end over the cover, which also leads to stress concentrations.